It isn't something I did per se, but it is a misconception that hugely influenced my behaviour.
As a kid, I was always under the intense impression that I was expected to know a lot of stuff, and that my own knowledge and skills were mediocre at best. It just kinda made sense that people would expect serious things from me at school and such. I was always mortified I would not be able to catch up.
For instance, I was able to read fluently by the time I was 4, and I could write in block letters, but not cursive. By the end of kindergarten, I thought that 7 was way too old to lack such a basic skill and I assumed that in elementary school, I'd be expected to write easily for extended periods of time.
On a wall in our kindergarten, individual handwritten letters' shapes were displayed on a poster or something like that. Before summer holidays of my last year before going to elementary school, I tried very hard to learn the shapes by heart, and when I couldn't (I didn't have that kind of dedication or attention span), I was sure I'd drop out of school in the first week or something.
To add to my assumption that I thought people knew what I knew (and more), I didn't ask when I didn't understand something. So. When the last day of the first year of school arrived, my mother jokingly asked me if I thought I'd advance to the next class, but she used a somewhat formal term that wasn't part of my vocabulary (doesn't translate too well into English, sorry). I didn't know what she meant, but I thought there was something special that I needed to do. So, being the best student in class and likely one of the best in the entire school, I waited for what felt like eternity for my teacher to give me my certificate of advancing to the next class. And since the teacher (wisely) felt that weaker students needed attention and praise just as much as smarter kids, I was literally the last kid. My mother didn't even know what kind of terror I had been in until years later.
I got over this kind of thinking in time, but it did persist for a long time, sometimes in ways that I kick myself for nowadays. I wanted to train martial arts since I was like 5. But I was a super-clumsy and nerdy kid. And of course I assumed, until I was 10 at least, that I'd be expected to be super fit and get into fights and who knows what else. So I never asked my parents to sign me up for a martial arts class, because I was afraid. On their part, my parents (who were always DESPERATE to find a physical activity for me to do, because I could pick up a book and start reading it in a middle of playing with other kids) didn't think that karate or judo would appeal to me at all. So they never thought to include it in a multitude of physical classes and activities that they tried to get me into.
To this day, I regret this lost opportunity. I did eventually take up martial arts and have been training ever since, but I started when I was like 20.